The present invention relates to the cooling of drawn fibers and particularly to the cooling of drawn optical glass fibers using a gaseous coolant. More particularly, the present invention relates to the use of a cap assembly for collecting cooling gas withdrawn from an optical glass fiber heat exchanger to minimize loss of coolant gas from the heat exchanger. The present invention also relates to the use of a cap assembly to supply gas to and withdraw gas from optical glass fiber heat exchanger.
Optical fibers are conventionally made from glass rods or preforms having a central core of glass enveloped by a cladding of glass whose refractive index is lower than that of the glass core. The fiber is produced by heating the glass preform to softening temperature in a furnace and drawing the fiber from the softened preform. The fiber is rapidly cooled sufficiently to enable a protective coating of resin material to be applied to the surface of the drawn fiber. The cooling is carried out by drawing the fiber through a heat exchanger wherein it comes into contact with a gaseous coolant.
The gaseous coolant is continuously passed through the heat exchanger in a direction which is cross-flow, counter-flow, co-current-flow, or combination mode thereof relative to the direction of the movement of the glass fiber through the heat exchanger. The gaseous coolant transfers heat from the glass fiber to the walls of the heat exchanger which is cooled by the surrounding atmosphere and/or a cooling medium, usually water, which flows through passages in the heat exchanger. The gaseous coolant is generally helium although other gases or mixtures can be used. Helium is the preferred coolant gas because of its favorable heat transport properties and is safe to use. However, helium is costly relative to other gases so it is desirable to capture the helium and recycle it for reuse in the heat exchanger.
If the exhausted helium gas from the heat exchanger is vented to the atmosphere as what has been done in the current fiber production processes, the fiber production cost would be disadvantageously higher especially as the draw speed of fiber has kept increasing over the last 30 years. In order to reduce the fiber cost associated with helium use in the fiber cooling step, helium recovery systems and apparatuses have been proposed to recover the helium gas. Exhausted helium gas with contaminants such as air and moisture is evacuated from the heat exchanger and purified before being recycled back to the coolant gas feed stream to the heat exchanger.
However, these helium recovery systems suffer from one or more of the following drawbacks:(1) fiber vibration as a result of the ambient air flow at the fiber inlet or outlet where the helium is collected and vacuum is applied; (2) fluctuation in fiber diameters due to lack of control in the pressure, composition, and flow rate of the recovery system, which would negatively affects the fiber""s mechanical and optical quality; (3) lower coolant (such as helium) purity and recovery due the ingress of air into the collected stream and egress of coolant (such as helium) from heat exchanger.
Air infiltration can be reduced significantly by ensuring there is a positive differential pressure between the cooling gas inside the heat exchanger and the surrounding environment. This has a disadvantage in that valuable helium will be lost to the environment through the fiber inlet end opening and/or fiber outlet end opening of the heat exchanger. Efforts have been made to minimize the amount of helium efflux and air influx through the fiber inlet end and outlet openings. For example, controlling the flow of helium into and out of the heat exchangers to limit air infiltration into the heat exchangers. However, operating heat exchangers at atmospheric or superatmospheric pressure results in significant loss of coolant gas from the system. A more economical processes for producing optical glass fiber are constantly sought.
The present invention provides a novel cap assembly for collecting the cooling gas with high purity and high recovery efficiency to reduce the cost of producing optical glass fiber without causing any negative impact on the fiber production process.
The present invention provides for a method of reducing the loss of gaseous coolant from an optical fiber cooling system by use of a cap assembly for collecting cooling gas from a heat exchanger. The cap assembly is designed to mount to the top, bottom or top and bottom of the heat exchanger and allow passage of the fiber through the cap assembly(s) and heat exchanger. The cap assembly also provides for at least one port means which can be used to supply or withdraw cooling and/or sealing gases depending on how and where the cap assembly is mounted on the heat exchanger. High coolant purity and recovery efficiency can be achieved with the device described in this invention without causing any negative impact on the fiber-drawing process and the air ingress and helium egress can be significantly minimized or even eliminated.
In one broad embodiment of the present invention, the invention comprises a method of cooling a hot drawn fiber in a heat exchange unit having one fiber inlet opening, one fiber outlet opening, at least one cooling gas inlet and at least one cooling gas outlet, and the cap assembly and a gas pumping means comprising the steps of passing the fiber through the heat exchanger, introducing gaseous coolant into the heat exchanger via at least one cooling gas inlet and withdrawing from the cap assembly via at least one gas outlet the collected helium stream mixed with contaminants which may be purified and recycled back into the coolant gas stream. Optionally, a sealing gas such as nitrogen, carbon dioxide, hydrogen, helium (purified or captured from the heat exchanger), argon, dry air or mixtures thereof may be introduced into the cap assembly thereby minimizing the infiltration of contaminants and egress of helium out of the heat exchanger.